1) Field of the Invention
The present invention relates to an electrode for an electric discharge surface treatment, and method and apparatus for the electric discharge surface treatment.
2) Description of the Related Art
A turbine blade of a gas turbine engine for an aircraft, for example, requires, on its surface, a coating or hardfacing with a material having strength and a lubrication property under a high-temperature environment. Such materials having the strength and the lubrication property include Cr (chromium) and Mo (molybdenum) of which the lubrication property is obtained when Cr and Mo are oxidized under a high-temperature environment. Conventionally, a material containing Cr or Mo is used to form a thick coating through a scheme such as welding or thermal spraying. Here, welding is a scheme using an electric discharge between a work to be processed (hereinafter, work) on which a coating is to be formed and a welding rod to fuse and attach the material of the welding rod onto the work. Thermal spraying is a scheme of spraying a metal material in a melted state onto a work to form a coating.
However, welding and thermal spraying are both manual operations and require a high skill level. Therefore, it is difficult to perform such operations on a production line, disadvantageously leading to an increase in cost. Moreover, since welding in particular is a scheme in which heat enters a work in a concentrated manner, when a thin material is processed or when a material that is easy to be broken, such as a single-crystal alloy or directionally-controlled alloy including a unidirectionally-solidified alloy, is processed, weld cracking is prone to occur, thereby disadvantageously reducing yield.
On the other hand, as another exemplary surface treating technique different from such surface treating schemes as welding and thermal spraying in which a coating with strength and a lubrication property under a high-temperature environment is formed on a work, a surface treatment through electric discharge machining (hereinafter, electric discharge surface treatment) has been established (for example, refer to a first patent document). In forming a thick coating through this electric discharge surface treatment, what has the most influence on coating performance is supply of a material from an electrode side, melting of the supplied material on a work surface, and bonding of the supplied material with a work material. What has an influence on the supply of the electrode material is strength, that is, hardness, of the electrode. In the first patent document described above, with an electrode for electric discharge surface treatment being provided with hardness to some extent, supply of an electrode material through electric discharge is suppressed, and the supplied material is sufficiently melted to form a hard ceramic coating on a work surface. Such a technology is disclosed, for example, in International Publication No. 99/58744 Pamphlet (pp. 7-8).
However, there is a problem in achieving a surface treatment for forming a coating with strength and a lubrication property under a high-temperature environment by using electric discharge surface treatment. As described above, Cr or Mo has to be oxidized under a high-temperature environment to become an oxide. When the material supplied between poles from the electrode by discharge energy is sufficiently melted and is processed in dielectric fluid, such as oil, containing carbon, however, the material forming a carbide is transformed to a carbide. Here, examples of materials that prone to form a carbide are Ti (titanium), V (vanadium), Cr, Zr (zirconium), Nb (niobium), Mo, Hf (hafnium), Ta (tantalum), W (tungsten), Si (silicon), and B (boron). When an electric discharge surface treatment is performed by using an electrode containing powders of such materials, the materials are transformed to carbides. For example, Ti is transformed to titanium carbide, Cr is transformed to chromium carbide, and Mo is transformed to molybdenum carbide. A carbide is resistant to oxidation compared with a metal. For example, since chromium carbide is resistant to oxidation up to a high temperature of approximately 900 degrees Celsius, chromium carbide resists becoming an oxide under a high-temperature environment and, consequently, the lubrication property cannot be exhibit efficiently.